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A unit configuration file whose name ends in ".target" encodes
information about a target unit of systemd, which is used for grouping units
and as well-known synchronization points during start-up.

This unit type has no specific options. See systemd.unit(5)
for the common options of all unit configuration files. The common
configuration items are configured in the generic [Unit] and [Install]
sections. A separate [Target] section does not exist, since no
target-specific options may be configured.

Target units do not offer any additional functionality on top of
the generic functionality provided by units. They exist merely to group
units via dependencies (useful as boot targets), and to establish
standardized names for synchronization points used in dependencies between
units. Among other things, target units are a more flexible replacement for
SysV runlevels in the classic SysV init system. (And for compatibility
reasons special target units such as runlevel3.target exist which are used
by the SysV runlevel compatibility code in systemd. See
systemd.special(7) for details).

The following dependencies are added unless DefaultDependencies=no is
set:

•Target units will automatically complement all
configured dependencies of type Wants= or Requires= with
dependencies of type After= unless DefaultDependencies=no is set
in the specified units. Note that Wants= or Requires= must be
defined in the target unit itself — if you for example define
Wants=some.target in some.service, the automatic ordering will not be
added.

When adding dependencies to other units, it's important to check
if they set DefaultDependencies=. Service units, unless they set
DefaultDependencies=no, automatically get a dependency on
sysinit.target. In this case, both emergency.target and
systemd-networkd.service have DefaultDependencies=no, so they are
suitable for use in this target, and do not pull in sysinit.target.

You can now switch into this emergency mode by running
systemctl isolate emergency-net.target or by passing the option
systemd.unit=emergency-net.target on the kernel command line.

Other units can have WantedBy=emergency-net.target in the
[Install] section. After they are enabled using systemctl
enable, they will be started before emergency-net.target is
started. It is also possible to add arbitrary units as dependencies of
emergency.target without modifying them by using systemctl
add-wants.